1. Field of the Invention
The present invention relates to a pressure-applying fixing roller (hereinafter referred to as a “fixing pressure roller”) and to a fixing device having the fixing pressure roller.
2. Background Art
A fixing unit of a copying machine or a printer employs a roller, and examples of the roller include a roller made of a metallic (iron, aluminum, etc.) core and an elastic layer (silicone rubber) (hereinafter referred to as fixing roller), and a roller made of a metallic core, an elastic layer, and a release layer (fluororesin tube) (hereinafter referred to as a pressure roller). Generally, the elastic layer of such a fixing pressure roller is made of foamed silicone rubber. Foamed silicone rubber has low hardness and ensures a wide fixation width, and has such a low heat capacity that the heat of a heat source built in a fixing roller or a fixing belt facing the elastic layer is not readily transferred thereto.
Such foamed silicone rubber is generally produced via a chemical foaming process, in which a foaming agent is incorporated in advance into a base rubber, and the foaming agent is gasified through heating. However, difficulty is encountered in controlling the foaming process during rubber curing, and provided cells are not uniform in size and shape, which is problematic. Thus, a further problem arises in that breakage of cells (foams) proceeds, to thereby cause poor hardness. That is, the produced silicone rubber foam has problematically insufficient durability. From another aspect, in a recent trend of reduction in particle size of toner, such foamed silicone rubber is not sufficient in realization of enhancement in quality of printed images and in high resolution. Thus, these problems remain to be solved.
One technique proposed to solve the problems is forming microcells by pressurizing a gas such as carbon dioxide under a pressure higher than the critical pressure thereof, to thereby disperse the gas in rubber, and by rapidly reducing the pressure to provide foam (see Patent Documents 1 and 2).
However, the technique also has a problem. Foam provided in unvulcanized rubber is released with the lapse of time, and therefore, difficulty is encountered in setting and controlling the curing time and other curing conditions. In some cases, most of the foam disappears.
In order to solve the above problem, there has been proposed a technique in which two or more curing agents having different reacting temperatures are added to a rubber base, and the rubber base is pressurized in a semi-vulcanized state while one curing agent has been consumed. In this case, no bubbles remain in the cured portion, and difficulty is encountered in realizing low hardness and low heat capacity.
Thus, there has been proposed another technique for attaining small cell diameter and low hardness, the technique including controlling the degree of plasticity (hereinafter referred to as “plasticity degree”) of unvulcanized rubber (see Patent Documents 3 and 4).
However, an unvulcanized rubber whose plasticity degree has been modified is difficult to mold. In addition, curing rate of the rubber and decomposition rate of the organic foaming agent must be precisely regulated.
Under such circumstances, there is demand for a technique for readily molding foamed silicone rubber and providing small cells while the rubber maintains low hardness and low heat capacity.
Patent Document 1: Japanese Patent No. 3506543
Patent Document 2: Japanese Patent No. 3642890
Patent Document 3: Japanese Patent No. 4342229
Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 2003-345158